The invention relates to a method that is intended for determining and/or predicting the power capacity of a battery and that uses a model of the battery, based on an equivalent circuit diagram, that predicts the power capacity of the battery. Such a method is known from DE 1 033 7064 B4.
The aim is to use this known method to predict the high current carrying capacity of a battery, in particular, a starter battery for a motor vehicle. However, this prognosis of the starting capability does not enable the ability to make a statement about the power capacity during the rest of the operating period of the battery. However, such a statement is absolutely mandatory for operating the battery in an electric and/or hybrid vehicle. Only after the prediction of the power that can be provided by the battery in a short period of time without violating the specified voltage and current limits is it possible to use the battery in such a vehicle.
It is known from the prior art to determine the power capacity of a battery by use of characteristic maps of the voltage of the battery under load. The input parameters for these voltage characteristic maps for a specific charge or discharge power is the respective state of the battery (temperature, pulse duration, state of charge and/or open circuit voltage). See the symposium report by Bohlen, O.; Gerschler, J. B.; Sauer, D. U.; Birke, P. and Keller, M., “Robust algorithms for a reliable battery diagnosis—managing batteries in hybrid electric vehicles,” EVS, Internat. Electric Vehicle Symp., 22, 2006, 2010-2021. Then, the power specifications are made available to an energy management system of, for example, a hybrid or battery powered vehicle.
An additional method that is known from the prior art consists of storing the equivalent internal resistance values of the battery for specified prognosis periods in characteristic maps or to determine the equivalent internal resistance values when the battery is operating (DE 10205120A). Then, taking into account the instantaneous open circuit voltage and an assumed load at constant current, the equivalent internal resistance values are used to calculate a voltage drop that is then compared with a specified limit.
The prior art methods do not consider the limit for the maximum charge voltage and the minimum discharge voltage that depends on the respective type of battery.
The power specifications stored in the characteristic maps do not map the dynamic behavior of batteries. As a result, the power values are either too low or too high over a specified prognosis period. In the first case the result is an overload of the battery, and in the second case the result is an unnecessary over-sizing of the battery. In addition, the number of prognosis periods is affected directly proportional to the memory space requirement, for example, during a microprocessor based calculation.
In addition, the prior art methods do not consider the internal state of the model that describes the influence of the load history of the battery at the beginning of the prognosis. Similarly the methods do not differentiate between the voltage limited and the current limited case.
The object of the present invention is to provide a method that delivers a realistic prognosis of the maximum power that can be provided by a battery.
This and other objects are achieved by a method for determining and/or predicting the maximum power capacity of a battery using a model of the battery, based on an electric equivalent circuit diagram that predicts the maximum power capacity of the battery. The maximum power of the battery is predicted for a defined prognosis period and for different operating modes with respect to the charging or discharging operation and taking into consideration the maximum allowable operating voltage and the maximum allowable operating current.
The present invention differentiates between four cases in total:
1) charging with voltage limitation,
2) charging with current limitation,
3) discharging with voltage limitation, and
4) discharging with current limitation,
where the terms “current limitation” and “voltage limitation” are defined as the consideration of the battery type dependent maximum value for the respective electric load (current or voltage).
Only after the inventive differentiation between the current limited and the voltage limited case does the battery model deliver realistic statements about the expected maximum power capacity of the battery over the entire prognosis period.
If, at the same time, the power capacity is calculated during the charging operation, taking into consideration the maximum charge current and the maximum charge voltage, then the result is usually two different values for the maximum power capacity. An advantageous further development of the invention consists of using the smaller (in terms of amount) value of the two values. As a result, none of the allowable operating limits of the battery are exceeded. The same applies to the discharging operation.
An additional improvement of the invention consists of taking into consideration the prehistory of the battery by use of an output voltage value. After a prolonged charging phase, the double layer capacitor of the battery is already polarized. Therefore, less charge power can be consumed at the same charge voltage than if the battery was previously at rest or was discharged. The same applies to the discharge case.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.